This invention relates to flexible polyurethane foam, and particularly to flexible polyurethane foam prepared from a relatively high equivalent weight polymer of propylene oxide.
It is well-known to prepare flexible polyurethane foam by reacting a polyether polyol with a polyisocyanate in the presence of a blowing agent. The polyether polyols most commonly used for this purpose are polymers and copolymers of propylene oxide. Polyurethane foams prepared from these poly(propylene oxide) polymers have many excellent properties, and thus are made and used in very substantial quantities.
It would, however, be desirable to improve certain aspects of these foams. For example, foams prepared from poly(propylene oxide) polymers often lack "green strength", i.e., adequate physical strength after initial curing. This property is quite important upon demolding the article, since its ultimate physical properties usually do not develop until several days after its production. Nonetheless, it is usually necessary to handle the foam immediately after its preparation. For example, it is usually necessary to move the foam to a storage, packout or shipping area, or to fabricate the foam for a particular application. In the case of molded foam, it is necessary to demold the foam so the mold can be re-used. During these operations, the foam must have sufficient strength to withstand a significant amount of handling without tearing, becoming distorted or sustaining non-recoverable blemishes.
In addition, these foams quite often have high "compression sets". Compression set is a measure of how well a foam recovers its initial dimensions when compressed. Ideally, flexible foam should regain essentially its original dimensions after being compressed. In practice, however, foam prepared from poly(propylene oxide) polymers tend to lose from about 5 to about 50% or more of their uncompressed height during standard tests for compression set.
Resolution of these problems is made more difficult because modifications which tend to ameliorate one of these problems usually exacerbate other important foam properties. For example, green strength can often be improved by increasing the reactivity of the poly(propylene oxide) polymer through ethylene oxide end-caps. However, increasing the ethylene oxide content of the polymer increases the foam's sensitivity to moisture, which in turn often causes greater compression sets.
It would be desirable to provide a means whereby flexible polyurethane foam could be prepared with better processing characteristics and improved physical properties, particularly improved compression set.